The two sides said three members of the Syrian security services used to clamp down on anti-government protests were killed in and around Damascus Tuesday, and four people were injured when a bomb exploded outside an Iranian culture and travel center in the capital, The New York Times reported.

An intelligence officer was shot and killed Tuesday in Barzeh, a northern suburb of Damascus, the British-based Syrian Observatory for Human Rights said. The man was suspected of identifying protesters to government forces for arrest.

A retired lieutenant colonel and his brother, also an officer, were assassinated in Damascus Tuesday, Syria's official news agency said, blaming "terrorists" for the attack.

Opposition activists, meanwhile, said Syrian forces have followed a predictable pattern since the first wave of U.N. monitors arrived last week: Government soldiers are largely quiet while around the monitors but resume their attacks once the monitors leave a community.

About 12 monitors have been sent to the country so far. Despite the cease-fire -- part of a six-point peace plan proffered by U.N.-Arab League envoy Kofi Annan -- violent attacks have taken place since plan went into effect April 12.

Annan, who briefed the Security Council Tuesday, said Syrian army attacks after the monitors departed on people who staged anti-government protests while monitors were in the cities is "unacceptable and reprehensible, if true."

"We have credible reports that when they [monitors] leave, the exchanges start again, that these people who approach the observers may be approached by the Syrian security forces or the Syrian Army or even worse, perhaps killed, and this is totally unacceptable," Annan's spokesman, Ahmad Fawzi, told U.N. television.

The U.N. Security Council backs the peace plan, with the full contingent of 300 inspectors expected to be sent to Syria during the next few months, the Times reported.

Susan E. Rice, the U.S. ambassador to the United Nations, said Tuesday the monitors' presence already has reduced the violence and the goal is to get all monitors deployed to Syria as quickly as possible.

However, Syria rejected one monitor based on his nationality and told the United Nations it won't accept any monitors from "Friends of Syria" countries, an informal organization of about 70 countries backing the opposition as it seeks political change.

"The onus remains on the Syrian government to halt the violence," Rice said, "then subsequently on both sides to maintain a cessation of violence."

The United Nations estimates at least 9,000 people have died since the protests against the government of President Bashar Assad began in March 2011. Activist groups estimate more than 11,000 people have died.

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